The present invention relates to a high-voltage pulse generating circuit for use in discharge-excited lasers such as copper vapor lasers, excimer lasers, etc. and accelerators such as linear induction accelerators, and more particularly to a high-voltage pulse generating circuit comprising a magnetic pulse compression circuit.
Discharge-excited lasers such as copper vapor lasers, excimer lasers, etc. are expected to be used for uranium enrichment, lithography, etc.
Such discharge-excited lasers are required to have large output, high pulse-repetition rate, high reliability and long service life. For achieving these requirements, a high-voltage pulse generating circuit as shown in FIG. 4 is used. This high-voltage pulse generating circuit comprises a variable high-voltage dc power supply 1, a resistor 2 for charging a main capacitor 5, a thyratron 3, an inductor 4, a capacitor 6, main laser discharge electrodes 7, a saturable reactor 8, an inductor 9 for charging the main capacitor 5, a peaking capacitor 10, an output winding 11 of the saturable reactor 8, a reset winding 12 for the saturable reactor 8, and a reset circuit 14 for the saturable reactor 8. The reset circuit 14 has output terminals 15, 16 connected to the terminals of the reset winding 12 of the saturable reactor 8.
Explanation will be made referring to FIGS. 4, 9 and 10 on the operation of this circuit when parameters of the constituent elements are optimized such that an energy transmission efficiency from the main capacitor 5 to the peaking capacitor 10 is maximum.
Incidentally, in the circuit shown in FIG. 4, the reset circuit 14 of the saturable reactor 8 has a structure shown in FIG. 5. In FIG. 5, 17 denotes a dc power supply, 18 a resistor, and 19 an inductor for absorbing surge voltage.
In the turn-off period of the thyratron 3, the saturable reactor 8 is reset from a point "e" to a point "a" in FIG. 9, by a magnetizing force Hr generated by current for charging the main capacitor 5 which flows through a course from a positive electrode of the dc power supply 1 to the resistor 2, the inductor 4, the main capacitor 5, the output winding 11 of the saturable reactor 8, the inductor 9 and a negative electrode of the dc power supply 1, and a reset current Ic which flows from the reset circuit 14 to the reset winding 12 of the saturable reactor 8.
Next, when the thyratron 3 is turned on at t=0 in FIG. 10, terminal voltage v.sub.6 of the capacitor 6 increases, as shown in FIG. 10(a), in the polarity shown in FIG. 4, by discharge current i.sub.1 shown in FIG. 10(b) which flows through a course from a positive electrode of the main capacitor 5, to the inductor 4, the thyratron 3, the capacitor 6 and a negative electrode of the main capacitor 5. During this period, the magnetic flux density of the saturable reactor 8 changes from a point "a" toward a point "b" in FIG. 9. At this time, since the output winding 11 of the saturable reactor 8 has an extremely large inductance L.sub.11 (unsat.), current i.sub.2 flowing through a course from the capacitor 6 to the capacitor 10, the output winding 11 of the saturable reactor 8 and the capacitor 6 is extremely smaller than the current i.sub.1 as shown in FIG. 10(e). Thus, the saturable reactor 8 is in a turn-off state equivalently. Therefore, as shown in FIG. 10(c), the output winding 11 of the saturable reactor 8 blocks the voltage at a polarity shown in FIG. 4.
When the current i.sub.1 becomes zero at t=.tau..sub.1, the magnetic flux density of the saturable reactor 8 reaches a point "b" in FIG. 9, so that a magnetic core of the saturable reactor 8 is saturated. At this time, the output winding 11 of the saturable reactor 8 has inductance L.sub.11 (sat.) sufficiently smaller than the inductance of the inductor 4, so that most of charges stored in the capacitor 6 flow as current i.sub.2 in the direction shown in FIG. 4. As shown in FIG. 10(e), i.sub.2 drastically increases, so that the magnetic flux density of the saturable reactor 8 changes from a point "b" to a point "Br" via a point "c" in FIG. 9. Accordingly, energy stored in the capacitor 6 is mostly transmitted to the peaking capacitor 10 as shown in FIG. 10(d).
Incidentally, a period from the turn-on of the thyratron 3 and to a time at which the current i.sub.2 becomes zero is called "gate period." Assuming that each element suffers from no loss, ##EQU1## E: Input dc power supply voltage (V). N.sub.11 : Number of winding of output winding 11 of saturable reactor 8.
Ae: Effective cross section (m.sup.2) of saturable reactor 8. PA0 .DELTA.B.sub.m : Operating magnetic flux density (T) of saturable reactor 8. PA0 Bs: Saturation magnetic flux density (T) of saturable reactor 8. PA0 Br: Residual magnetic flux density (T) of saturable reactor 8. PA0 L.sub.4 : Inductance (H) of inductor 4. PA0 L.sub.11(sat) : Inductance (H) of output winding 11 of saturable reactor 8. PA0 C.sub.5 : Capacitance (F) of main capacitor 5. PA0 C.sub.6 : Capacitance (F) of capacitor 6. PA0 C.sub.10 : Capacitance (F) of peaking capacitor 10. PA0 H.sub.LM : Gate magnetizing force of saturable reactor 8. PA0 I.sub.2m : Wave height (A) of i.sub.2. PA0 le: Mean magnetic path length (m) of saturable reactor 8.
As soon as all the energy of the capacitor 6 is transmitted to the peaking capacitor 10, the main laser discharge electrodes 7 are broken down at a time of .tau..sub.1 +.tau..sub.2 as shown in FIG. 10, so that the energy of the peaking capacitor 10 is consumed in a laser gas. At this time, although most energy accumulated in the peaking capacitor 10 is consumed in a laser gas via the main laser discharge electrodes 7, a part of the energy is used to reset the saturable reactor 8. By this energy, the magnetic flux density of the saturable reactor 8 changes from a point "Br" to a point "e" via a point "d" in FIG. 9.
The above operation is usually repeated at a predetermined pulse-repetition rate.
Incidentally, the reset circuit 14 functions to reset the saturable reactor 8 to a magnetic flux density smaller than Br, even when the current discharged from the main capacitor 5 is smaller than current necessary for generating a full-reset magnetizing force Hr of the magnetic core of the saturable reactor 8. The details of the reset circuit are described in Japanese Patent Laid-Open No. 63-171172, etc.
In the above conventional circuit, there is one magnetic pulse compression circuit comprising a saturable reactor, but some high-voltage pulse generating circuits comprise multistage magnetic pulse compression circuits consisting of a plurality of magnetic pulse compression circuits each comprising a saturable reactor. Also, in the case of accelerators such as linear induction accelerators, high-voltage pulse generating circuits comprising multistage magnetic pulse compression circuits are mostly used because large output is required.
Incidentally, the principle of a magnetic pulse compression circuit is described in "The Use of Saturable Reactors As Discharge Devices for Pulse Generators," W. S. Melville, Proceedings of Institute of Electrical Engineers, (London) Vol. 98, Part 3, No. 53, pp. 185-207 (1951); the application of such circuit to discharge-excited lasers is described in "Electrical Excitation of an XeCl Laser Using Magnetic Pulse Compression," I. Smilanski, S. R. Byron and T. R. Burkes, Appl. Phys. Lett. 40 (7), pp. 547-548 (1982); the magnetic pulse compression circuit using semiconductor elements is described in U.S. Pat. No. 4,549,091, and "An Efficient Laser Pulser Using Ferrite Magnetic Switches," H. J. Baker, P. A. Ellsmore and E. C. Sille, J. Phys. E. Sci. Instrument 21 (1988), pp. 218-224.
Also, in accelerators such as linear induction accelerators for free electron lasers, etc., high-voltage pulse generating circuits having the same system as described above may be used. The details are described, for instance, in D. Birx, E. Cook, S. Hawkins, S. Poor, L. Reginato, J. Schmidt and M. Smith: "The Application of Magnetic Switches as Pulse Sources for Induction Linacs", IEEE Transactions on Nuclear Science, Vol. NS-30, No. 4, pp. 2763-2768 (1983), and U.S. Pat. No. 4,730,166.
In discharge-excited lasers, the stabilization of laser output and the reduction of jitter are required. For instance, in excimer lasers for lithography, it is necessary to stably supply a laser output of about 100 mJ per one pulse for a period of 10.sup.8 shots or more in a pulserepetition rate of about 500 Hz. However, since a laser gas is deteriorated by repeated operation, it is necessary to gradually increase an energy to be supplied to the laser gas, in order to satisfy the above output requirements. For this purpose, in the conventional circuit shown in FIG. 4, the input dc power supply voltage is gradually increased. In the circuit shown in FIG. 4, since the operating magnetic flux density (.DELTA.B.sub.m expressed by the formula (4)) of the saturable reactor 8 in a gate period is constant, voltage and current at main elements in the circuit have waveforms shown in FIG. 11, when the input dc power supply voltage is lower than an optimum value at which the energy transmission efficiency from the main capacitor 5 to the peaking capacitor 10 is maximum. On the other hand, when the input dc power supply voltage is higher than the above optimum value, the voltage and current waveforms become as shown in FIG. 12. In both cases, the energy transmission efficiency from the main capacitor 5 to the peaking capacitor 10 decreases, and after-current of the current i.sub.1 flowing between main electrodes of the thyratron 3 increases, causing inverse current to flow. As a result, the loss of the thyratron 3 increases. Further, since a percentage of energy which does not contribute to the laser oscillation increases in the laser gas, the service life of the laser gas decreases. Therefore, the number of shots by which a constant laser output can be obtained is limited to 10.sup.6 or so.
In the copper vapor lasers used in a uranium enrichment process, stable, continuous operation is required at a pulse-repetition rate of about 5 kHz or more and at a laser output of about 100 W with a jitter of .+-.3 nanoseconds or less for about 1000 hours or more. Since such lasers are operated at a pulse-repetition rate about one order higher than that of the excimer laser, it is strongly desired to use a high-voltage pulse generating circuit comprising a multistage magnetic pulse compression circuit and semiconductor elements such as thyristors instead of thyratrons as switching elements. However, in the conventional high-voltage pulse generating circuit utilizing a multistage magnetic pulse compression circuit, to optimize the energy transmission efficiency from the main capacitor to the peaking capacitor at a final stage, inductance in each magnetic pulse compression circuit should be adjusted. For this purpose, an inductor is inserted in series to a saturable reactor in each magnetic pulse compression circuit to measure a pulse width of current flowing after the saturation of the saturable reactor, and then an inductor having a different inductance is inserted in series to the saturable reactor. This is because the operating magnetic flux density of the saturable reactor constituting each magnetic pulse compression circuit in a gate period is constant as .DELTA.B.sub.m in the above formula (4). In addition, the above procedure should be utilized in the adjustment of the magnetic pulse compression circuit in the synchronous operation of a plurality of high-voltage pulse generating circuits, so that it is extremely difficult to use such system in commercial plants needing the synchronous operation of a plurality of high-voltage pulse generating circuits.
In free electron lasers or linear induction accelerators used for plasma heating of nuclear fusion plants, a kind of transformer for accelerating electron beams, which is called "accelerator cell," should be supplied with rectangular pulses having a voltage wave height of several hundreds of kV, a current wave height of several tens of kA and a pulse width of about 100 nanoseconds, with jitter within several nanoseconds at a pulse-repetition rate of several kHz or more in a burst mode for as long a period of time as possible. In the high-voltage pulse generating circuit in these applications, a multistage magnetic pulse compression circuit comprising thyratrons as switching elements in parallel is used. In this high-voltage pulse generating circuit, there is a problem that the energy transmission efficiency decreases as the operation time passes, since the operating magnetic flux density of the magnetic core of the saturable reactor in a gate period decreases by repeated operation because of temperature rise caused by the loss of the saturable reactor.